1. Field of the Invention
The present invention relates to a voltage controlled oscillator, and more particularly, it relates to a voltage controlled oscillator comprising a triplate type dielectric resonator and a method of regulating its oscillation frequency.
2. Description of the Background Art
A voltage controlled oscillator forms a circuit shown in FIG. 4, for example. The voltage controlled oscillator shown in FIG. 4 includes an oscillating part 1 and a buffer part 2. In the oscillating part 1, which includes an amplifier circuit 3 and a feedback circuit 4, an output from the amplifier circuit 3 partially feeds back by means of the feedback circuit 4, to cause oscillation. The oscillation frequency of this voltage controlled oscillator is determined by the circuit constant of the feedback circuit 4.
In a field other than the voltage controlled oscillator, a feedback circuit is generally formed by inductive reactance and capacitive reactance. In the voltage controlled oscillator, however, the feedback circuit 4 employs a dielectric coaxial resonator 5 which can attain a high Q-value (which is an index of the selectivity of the resonance frequency) in place of the inductive reactance and the capacitive reactance. A variable-capacitance diode 6 is coupled in parallel with the resonator 5 to adjust the capacitive reactance, thereby allowing variation of the oscillation frequency.
Referring to FIG. 5, an exemplary mechanical structure of a conventional voltage controlled oscillator is now described. This voltage controlled oscillator includes a circuit board 7. The circuit board 7 includes an insulating substrate of ceramic, for example, while an interconnection pattern (not shown) is formed on an upper major surface of the insulating substrate and an earth pattern is formed substantially entirely over a lower major surface thereof.
The dielectric coaxial resonator 5 described above with reference to FIG. 4 is arranged on the upper major surface of the circuit board 7, to be connected to the aforementioned interconnection pattern. The dielectric coaxial resonator 5 comprises a dielectric body 8. This dielectric body 8 is provided with a through hole 9 having a circular section and extending along its axial direction. The dielectric body 8 is in the form of a square pole whose section perpendicular to the axial direction is square-shaped. Each edge of the square forming the section of the dielectric body 8 is selected to have a length of 6 mm, for example. An inner peripheral conductor 10 is applied onto an inner peripheral surface of the dielectric body 8 defining the through hole 9, while an outer peripheral conductor 11 is applied onto an outer peripheral surface of the dielectric body 8. Further, a short-circuit conductor (not shown) is applied onto a left end surface of the dielectric body 8 in FIG. 5 for short-circuiting the inner peripheral conductor 10 and the outer peripheral conductor 11. A right end surface of the dielectric body 8 in FIG. 5 is in an open state with no application of a conductor. A lead 12 is arranged on the side of this open end surface so that its first and second ends are connected to the inner peripheral conductor 10 and the interconnection pattern formed on the circuit board 7 respectively.
This dielectric coaxial resonator 5 has a relation of L=.lambda.g/4, where L represents its axial length and .lambda.g represents the wavelength in its coaxial line. The resonance frequency of the dielectric coaxial resonator 5 is found by this equation.
Discrete electronic components 13a, 13b, 13c, . . . such as a transistor, a capacitor, and a variable-capacitance diode are arranged on the upper major surface of the circuit board 7 to be connected to the interconnection pattern. A metal case 14 is mounted on the circuit board 7, to cover the dielectric coaxial resonator 5 and the electronic components 13a, 13b, 13c, . . . The circuit board 7 is placed on a flat base 15, so that the earth pattern provided on the lower major surface of the circuit board 7 is connected to this base 15. The base 15 is provided with downwardly extending earth terminals 16a and 16b. On the other hand, the circuit board 7 is provided with a plurality of downwardly extending terminals 17a, 17b, 17c, 17d, . . . , which pass through the base 15 while being electrically insulated from this base 15, to extend in parallel with the earth terminals 16a and 16b. Some of the terminals 17a, 17b, 17c, 17d, . . . are adapted to be input and/or output signals, while the other ones are adapted to supply a voltage.
The oscillation frequency of this voltage controlled oscillator is regulated in the following manner: The dielectric coaxial resonator 5 can be regarded as forming a parallel resonance circuit of capacitive reactance formed between the inner and outer peripheral conductors 10 and 11 and inductive reactance formed along the longitudinal direction of the inner and outer peripheral conductors 10 and 11. Therefore, it is possible to increase the resonance frequency of this parallel resonance circuit by partially trimming the outer peripheral conductor 11 from its open end surface toward the short-circuit conductor. On the other hand, it is possible to reduce the resonance frequency by partially trimming the short-circuit conductor. Thus, the resonance frequency of the dielectric coaxial resonator 5 can be varied to regulate the oscillation frequency of the voltage controlled oscillator.
As hereinabove described, the dielectric coaxial resonator 5 has relatively wide end surfaces with each edge measuring 6 mm, for example. Therefore, the height of the dielectric coaxial resonator 5 hinders the voltage controlled oscillator from being reduced in height. In order to enable reduction in height of the voltage controlled oscillator, the dielectric coaxial resonator may be reduced in height to 1.5 mm, for example. However, such miniaturization of the dielectric coaxial resonator leads to reduction in the Q-value.
Further, the dielectric coaxial resonator 5 having an open end surface is easily influenced by the metal case 14, which degrades its electric characteristics such as the frequency. Particularly in a half-wave dielectric coaxial resonator (not shown) which resonates on the basis of the relation L=.lambda.g/2, such electric characteristics are remarkably influenced by the metal case since both end surfaces of the resonator are in open states.
In recent years, the discrete electronic components 13a, 13b, 13c, . . . have been increased in packaging density on the circuit board 7. In this case, it may be difficult to trim the outer conductor 11 or the short-circuit conductor in order to regulate the resonance frequency of the dielectric coaxial resonator 5.